LibWeb: Split StyleComputer work into two phases with separate outputs

Before this change, StyleComputer would essentially take a DOM element,
find all the CSS rules that apply to it, and resolve the computed value
for each CSS property for that element.

This worked great, but it meant we had to do all the work of selector
matching and cascading every time.

To enable new optimizations, this change introduces a break in the
middle of this process where we've produced a "CascadedProperties".
This object contains the result of the cascade, before we've begun
turning cascaded values into computed values.

The cascaded properties are now stored with each element, which will
later allow us to do partial updates without re-running the full
StyleComputer machine. This will be particularly valuable for
re-implementing CSS inheritance, which is extremely heavy today.

Note that CSS animations and CSS transitions operate entirely on the
computed values, even though the cascade order would have you believe
they happen earlier. I'm not confident we have the right architecture
for this, but that's a separate issue.

Libraries/LibWeb/DOM/Element.cpp

@@ -101,8 +101,10 @@ void Element::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_class_list);
     visitor.visit(m_shadow_root);
     visitor.visit(m_custom_element_definition);
+    visitor.visit(m_cascaded_properties);
     if (m_pseudo_element_data) {
         for (auto& pseudo_element : *m_pseudo_element_data) {
+            visitor.visit(pseudo_element.cascaded_properties);
             visitor.visit(pseudo_element.layout_node);
         }
     }
@@ -2273,6 +2275,28 @@ size_t Element::attribute_list_size() const
     return m_attributes->length();
 }
 
+GC::Ptr<CSS::CascadedProperties> Element::cascaded_properties(Optional<CSS::Selector::PseudoElement::Type> pseudo_element) const
+{
+    if (pseudo_element.has_value()) {
+        auto pseudo_element_data = get_pseudo_element(pseudo_element.value());
+        if (pseudo_element_data.has_value())
+            return pseudo_element_data->cascaded_properties;
+        return nullptr;
+    }
+    return m_cascaded_properties;
+}
+
+void Element::set_cascaded_properties(Optional<CSS::Selector::PseudoElement::Type> pseudo_element, GC::Ptr<CSS::CascadedProperties> cascaded_properties)
+{
+    if (pseudo_element.has_value()) {
+        if (pseudo_element.value() >= CSS::Selector::PseudoElement::Type::KnownPseudoElementCount)
+            return;
+        ensure_pseudo_element(pseudo_element.value()).cascaded_properties = cascaded_properties;
+    } else {
+        m_cascaded_properties = cascaded_properties;
+    }
+}
+
 void Element::set_computed_css_values(Optional<CSS::StyleProperties> style)
 {
     m_computed_css_values = move(style);

Libraries/LibWeb/DOM/Element.h

@@ -13,6 +13,7 @@
 #include <LibWeb/Bindings/ElementPrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/Bindings/ShadowRootPrototype.h>
+#include <LibWeb/CSS/CascadedProperties.h>
 #include <LibWeb/CSS/CountersSet.h>
 #include <LibWeb/CSS/Selector.h>
 #include <LibWeb/CSS/StyleInvalidation.h>
@@ -172,7 +173,7 @@ public:
     // https://html.spec.whatwg.org/multipage/embedded-content-other.html#dimension-attributes
     virtual bool supports_dimension_attributes() const { return false; }
 
-    virtual void apply_presentational_hints(CSS::StyleProperties&) const { }
+    virtual void apply_presentational_hints(GC::Ref<CSS::CascadedProperties>) const { }
 
     void run_attribute_change_steps(FlyString const& local_name, Optional<String> const& old_value, Optional<String> const& value, Optional<FlyString> const& namespace_);
 
@@ -189,6 +190,9 @@ public:
     void set_computed_css_values(Optional<CSS::StyleProperties>);
     CSS::StyleProperties resolved_css_values(Optional<CSS::Selector::PseudoElement::Type> = {});
 
+    [[nodiscard]] GC::Ptr<CSS::CascadedProperties> cascaded_properties(Optional<CSS::Selector::PseudoElement::Type>) const;
+    void set_cascaded_properties(Optional<CSS::Selector::PseudoElement::Type>, GC::Ptr<CSS::CascadedProperties>);
+
     void set_pseudo_element_computed_css_values(CSS::Selector::PseudoElement::Type, Optional<CSS::StyleProperties>);
     Optional<CSS::StyleProperties&> pseudo_element_computed_css_values(CSS::Selector::PseudoElement::Type);
 
@@ -410,11 +414,14 @@ private:
     GC::Ptr<DOMTokenList> m_class_list;
     GC::Ptr<ShadowRoot> m_shadow_root;
 
+    GC::Ptr<CSS::CascadedProperties> m_cascaded_properties;
+
     Optional<CSS::StyleProperties> m_computed_css_values;
     HashMap<FlyString, CSS::StyleProperty> m_custom_properties;
 
     struct PseudoElement {
         GC::Ptr<Layout::NodeWithStyle> layout_node;
+        GC::Ptr<CSS::CascadedProperties> cascaded_properties;
         Optional<CSS::StyleProperties> computed_css_values;
         HashMap<FlyString, CSS::StyleProperty> custom_properties;
     };